Microwave tuner having a rapid tuning rate



United States Patent O vs. Cl. sis-39.55 8 Claims ABSTRACT OF THEDISCLOSURE A tunable microwave cavity is provided having a movable wallfor changing the resonant frequency of a cavity. The microwave cavityincludes a dielectric window which is sealed to portions of the cavityto form a chamber, while the movable cavity wall is positioned outsideof this chamber confronting the dielectric window. Input and or outputcouplings are provided to introduce or extract microwave energy from thecavity. Moreover, in a preferred embodiment the cavity is designed tosupport a TE circular electric mode.

This invention relates to a microwave cavity tuner, and moreparticularly, to a rapid tuner adapted to tune the resonant outputcavity of a coaxial magnetron by positioning of a cavity wall.

As is known, piezoelectric devices exhibit a piezoelectric effect. Thatis, in response to the application of voltage, they expand and contract,and with more exotic constructions, flex in proportion to the magnitudeof a voltage applied across the electrodes of the device.

In the patent application, Ser. No. 636,540, filed May 5, 1967, ofWilliam H. Perkins and Robert E. Klotz, a piezoelectric actuator isdisclosed which positions the movable wall of a microwave cavity to forma microwave tuner. More particularly, a piezoelectric biomorph actuatorpositions the movable wall of the resonant output cavity of a coaxialmagnetron as a function of the magnitude of voltages applied across theelectrodes of the actuator.

The piezoelectric device disclosed in that application, and commonlytermed a piezoelectric sandwich or biomorph creates mechanical movement;that is, bends or flexes in response to the magnitude of appliedvoltages across the electrodes of the device. This mechanical movementis coupled to a movable microwave cavity wall to effect tuning of amicrowave cavity over a moderate bandwidth of frequencies at relativelyrapid rates.

As disclosed in that application, the microwave cavity is included in amagnetron. One portion of a piezoelectric biomorph is fixed to arelatively stationary portion of the magnetron housing, while anotherportion of the piezoelectric biomorph remains free to flex. A movablethin metallic layer forms a wall of a microwave cavity or resonantoutput cavity of a coaxial magnetron. This movable wall is mechanicallycoupled with the piezoelectric biomorph. Flexure or bending of thepiezoelectric biomorph which occurs as a function of voltages appliedacross the biomorph electrodes varies the position of the movable cavitywall; and hence, varies the size of the resonant output cavity.Accordingly, this varies the frequency of resonance of the resonantoutput cavity, and the frequency of oscillation of the magnetron.

The piezoelectric biomorph actuator is adapted for connection to asource of sweep voltages. This causes periodic flexing of the actuatorwhich reciprocates the movable cavity wall through a plurality ofpositions. Accordingly, the frequency of resonance of the resonant output cavity and the frequency of oscillation of the magnetron sweepsthrough a predetermined bandwidth of q a frequencies.

Ip one embodiment of the invention, disclosed in the aforecitedcopending application, the piezoelectric biomorph actuator is locatedoutside the evacuated portions of the magnetron housing. Movement of thefree end of the biomorph actuator is transmitted by a rod and bellowsarrangement to a movable wall located inside the evacuated regions ofthe magnetron housing. The bellows is constructed of a metallic materialwhich requires a relatively large force to move the rod and contract thebellows. Additionally, because of the inherent rigidity of the bellowsand its mechanical inertia, the inerti'a of the rod members therein, thesweep or speed rate at which the movable wall is positioned is limitedto a rate below that capable with a piezoelectric biomorph alone asdisclosed in other embodiments in that application. To compensate forthese defects, a plurality of piezoelectric biomorphs are connected inparallel or ganged in order to provide a sufiicient amount of forcenecessary to move the rod and bellows structure.

In other embodiments disclosed in said copending application, thepiezoelectric device is mounted within the evacuated or vacuum portionsof the magnetron housing, which avoids the response speed limitationsimposed by the bellows and rod arrangement. However, mounting thepiezoelectric biomorph within the evacuated magnetron housing involvesother complications in practice. First, should the piezoelectricactuator and supports require adjustment, the magnetron must bedisassembled, reassembled, and re-evacuated; an obviously expensiveproposition. Second, the elements of the piezoelectric actuator are ofnecessity subjected to a high vacuum. Since the relatively inexpensivecommercially available piezoelectric biomorph devices are not intendedfor high vacuum applications, the elements used in the commercialconstruction do not incorporate the required low vapor pressurematerials. Consequently, the material, such as epoxy, used in theconstruction of commercial piezoelectric biomorph devices, could over aperiod of time, vaporize, which may spoil the vacuum, and some of thisvapor may condense on other elements with the magnetron housing, whichmay change the operating characteristics of the magnetron. Accordingly,a more expensive piezoelectric device is custom constructed of low vaporpressure material, and preferably utilized in the various tunersdisclosed in that application. Third, in some available procedures ofmagnetron construction, the magnetron is heated to high temperatureswhich are above the Curie temperature of the piezoelectric material, atemperature at which the material loses its piezoelectric properties.That construction procedure necessitates the additional step of applyinga polarizing current to the piezoelectric device in order to restore thedestroyed piezoelectric properties. Moreover, high temperature heatingof a piezoelectric device of the commercially available. variety causessome decomposition of elements, such as the aforementioned epoxy, andemission of an undesirable carbonizing vapor. As previously mentioned, avapor spoils the vacuum and creates undesirable carbon deposits upon theinternal elements of the magnetron.

Therefore, it is an object of the invention to provide a piezoelectrictuner that is not exposed to the high vacuum evacuated region ofmicrowave cavity.

It is a further object of the invention to provide a relativelyinexpensive piezoelectric tuner for a microwave cavity that is easilyaccessible for adjustment.

It is another object of the invention to provide a piezoelectric tunerfor a magnetron that is assembled subsequent to assembly and evacuationof the magnetron.

It is another object of the invention to provide an externalpiezoelectric tuner for a coaxial magnetron with out a bellows and rodarrangement.

It is a further object of the invention to provide a piezoelectric tunerfor a magnetron that consists in part of ordinary, inexpensive,commercially available piezoelectric biomorph devices of ordinaryconstruction.

In accomplishing the foregoing objects, a tuner struc' ture is presentedwhich is used to advantage with any type of tuner drive, slow or rapid,electrical or mechanical, pieelectric or otherwise that relies upon thepositioning of a movable cavity wall to tune the frequency of resonanceof a microwave cavity. Heretofore, and as disclosed in the aforecitedcopending application Ser. No. 636,540, a given frequency change orbandwidth of tuning required a certain positional movement of themovable cavity wall. Desirably with the structure of the presentinvention, only about one-fourth as much wall movement is required totune a microwave cavity in a coaxial magnetron over the same bandwidth.As is apparent, this feature provides first, a larger bandwidth oftuning for any fixed amount of cavity wall movement in any prior arttuner; and, second, the sweep rate increases since for a given bandwidthof tuning and a given speed of tuning or movement of the movable wall,the required bandwidth is tuned or swept over in a shorter period oftime.

Therefore, it is another object of the invention to increase the tuningrate of a tunable microwave cavity;

It is a further object of the invention to reduce the amount of movementof a movable cavity wall necessary in order to tune a microwave cavityover a given bandwidth of frequencies.

The invention is characterized by a microwave cavity which supports a TEcircular electric mode of oscillation, and which contains a movablewall. A chamber is formed within the microwave cavity, and a dielectricwindow permeable to microwaves and impermeable to air seals this cavity.The movable wall is positioned so that it confronts the dielectricwindow and, therefore, effectively reflects microwave energy between itssurfaces and the other walls of the cavity to form an effectivemicrowave cavity of a given size. Positioning means are connected to themovable wall to position the wall relative to the dielectric window;and, hence, to correspondingly tune the microwave cavity.

Further, in accordance with the invention, the chamber consists of theinner walls and anode of a coaxial magnetron, and the dielectric windowis of a washer shape to conform to the shape of the available spacebetween the pole piece and cylindrical walls of the magnetron.

Moreover, in accordance with the invention, the positioning meansincludes a piezoelectric device connected with the movable wall andwhich positions the movable wall in response to the magnitude ofvoltages applied to the device.

The foregoing and other objects and advantages become apparent from areading of the following description in view of the figure whichillustrates in section a partial view of the tunable coaxial magnetronembodying the invention.

The figure shows in section a partial view of a tunable coaxialmagnetron embodying the invention. Typical elements found in a coaxialmagnetron, such as the magnets enveloping the magnetron structure, thetube socket, remaining portions of the housings and mountings, andelectrical power supplies are omitted because they are conventional anddo not clarify the invention. The figure shows a housing which consistsof a first cup-like housing portion 1 having an inner washer shapedupper wall 2, and an cylindrical inner wall 3. Grooves 4 are includedwithin the sides of housing portion 1 for allowing installation of wellknown cooling fins, not illustrated. A portion of a microwave window 6is sealed within an opening in one portion of the cylindrical inner wall3. This window 6 consists of a washer shaped dielectric insulator whichis brazed to the surrounding portions of the window structure to permitthe coupling of microwave energy therethrough without permitting thepassage of air or other gases. A cylindrical wall 7 is seated at an endto a ring-like seal or joint 8 which is connected to a second seal orjoint 9. Seal 9 is, in turn, sealed to the lower cylindrical wallportion of the first cup-like housing portion 1.

A cylindrical pole piece 10 is mounted by a pole piece flange portion 11to cylindrical wall 7. A second hollow cylindrical pole piece 12 ismounted within an opening in the top portion of the cup-like housingmember 1 and protrudes beyond the surface of wall 2. Each of the polepieces 10 and 12 are conventionally constructed to ferromagneticmaterial to provide a path for magnetic flux coupled thereto from aconventional permanent or electromagnet, not illustrated, to establish amagnetic field across the gap between the ends of the pole pieces. Polepieces 10 contains a cylindrical tubular end portion of reduced diameterand pole piece 12 is tapered to a smaller diameter at the protruding endin order to concentrate the magnetic flux into a smaller region.Extending through the hollow pole piece is a cylindrical cathode 13. Asis conventional, a filament, not illustrated, is included within thehollow portion 14 of the cylindrical cathode 12 for heating the cathodematerial.

Alternatively, it is conventional to utilize a filament winding coatedwith electron emissive material in lieu of a separate cathode, shouldsuch be desirable Concentric with and surrounding the cathode iscylindrical anode 15 fastened and brazed in place, in this instance, tothe top wall 2 of the first cup-like housing portion 1. As isconventional, the anode contains a plurality of anode resonatorssurrounding the cylindrical cathode. In the illustrated magnetron, theanode 15 contains an inner surface or wall 16, and an outer surface orwall 17, and each anode resonator is formed thereon in the space betweentwo adjacent ones of the plurality of vanes. Two nonadjacent vanes 18and 19 of the plurality are illustrated. The vanes are equally spacedfrom each other around the cylindrical inner wall 16 of the anode 15,and about cylindrical cathode 13. Each vane extends from the inner wall16 of anode 15 to within a predetermined distance of the cathodesurface. This leaves a gap between the vanes and the cathode 13 commonlytermed the interaction region.

A plurality of elongated slots 20 extend through anode 15 to form apassage between alternate ones of the anode resonators to the spacesurrounding the outer wall 17, the resonant output cavity 27,hereinafter described, for allowing microwave energy to be coupledtherebetween.

The foregoing details are descriptive of the conventional coaxialmagnetron while the following describe the construction embodying theinvention.

A first cylindrical support 21 is seated upon the pole piece flangeportion 11. A second cylindrical support member 22 is seated around thepole piece 10, and to the outer rim of a washer shaped support disk 23mounted about and seated upon the end cylindrical pole piece 10 aboutits reduced diameter cylindrical end portion. The second cylindricalsupport member 22 supports a third cylindrical support member 24. Awasher-shaped dielectric window 25, constructed for example, of alumina,is seated along its outer rim to a groove along the outer edge of thefirst cylindrical support member 21, and is seated at its inner rimalong a groove at the outer end of the second cylindrical support member24. Each of the support members 21, 22, 23, and 24 and window 25 arebrazed or sealed in place so as to form a vacuumtight connectionpartitioning portions of the magnetron housing within body portion 1.Thus, an air tight evacuated chember is formed between walls 2 and 3,sup ports 21, 22, 23, and 24 and window 25.

A thin lightweight washer-shaped movably mounted or movable conductivewall 26 confronts microwave window 25 on the outside of the formedevacuated chamber. However, because microwave energy penetrates throughthe window, the conductive wall 26 forms an outer boundary for microwaveenergy emanating or contained within the formed chamber and forms onewall of a microwave cavity 27, which is the resonant output cavity ofthe coaxial magnetron.

This resonant output cavity 27 is formed essentially between the topwall 2 of the housing member, a portion of the inner cylindrical wall 3of the housing portion 1, the outer wall 17 of anode 15, and the movableor positionable wall 26, and is somewhat donut-like in shape. Thisresonant output cavity, as is conventional, supports a TE circularelectric mode of oscillation at the frequencies to which the magnetronis tunable.

As is apparent, there is a small clearance between cylindrical housingwall 3 and the cylindrical support member 21. This clearance opens intothe larger regions of the resonant output cavity 27, but is noteffectively a part of that cavity. The clearance is very small relativeto the wavelength of the dominant mode of microwave generated in theresonant output cavity 27; and, the clearance effectively appears as avery high impedance to such microwaves. Hence, the region in theclearance does not perform any substantial hinderance to the operationof the magnetron.

As is apparent, various types of actuators can position the movable wall26 to tune the resonant output cavity 27. In the coaxial magnetronillustrated the movable wall is shown to be positioned under the controlof both a piezoelectric biornorph actuator 28, and a mechanical orelectromechanical positioner 50.

The piezoelectric biornorph actuator 28 is a multilayered sandwich orbiornorph which contains a first layer of piezoelectric material 29, asecond layer of piezoelectric material 30', a middle conductive layer ofelectrode 31 between the two piezoelectric layers and two outerconductive layers or electrodes 32 and 33, one on the outer surface ofeach piezoelectric layer. In the commercial construction the middleelectrode is a thin brass shim fastened to the piezoelectric layers byepoxy, and the two outer electrodes are formed with a very thinmetalized layer fired on to the piezoelectric layers. In this manner theforegoing elements form a physically integral thin flexible element,exaggerated in dimension in the figure for clarity. Moreover, theparticular shape of the piezoelectric biornorph actuator is of a washershaped geometry, since, like the movable wall 26 and dielectric window25, it must conform to the available space between the cylindrical polepiece and anode and the larger diameter cylindricalwalls surrounding thepole piece and anode.

Moreover, each piezoelectric layer of the piezoelectric biornorph isoppositely electrically poled. That is, one layer is poled in adirection from the middle to one outer electrode, and the other layer ispoled in a direction from the outer electrode to the middle electrode,which is accomplished, in each instance, by applying a large polarizingvoltage between an outer electrode and the middle electrode. Subsequentapplication of the same sweep voltage across both layers cause one layerto expand and the other to contract. Since the layers are integrallyjoined, a flexing or warping action occurs much like the operation of abi-metallic thermostat. This construction magnifies the movementavailable from the expansion and contraction of a homogenous mass ofpiezoelectric material.

An annular strip, lip, or coupling member 34, as variously termed, isconnected to piezoelectric biornorph actuator 28 along the outer rim andto the movable wall 26 along the outer rim thereof to enable movement ofthe actuator to be coupled to the movable wall. In the figure, lip 34 isformed integrally with movable wall 26 by bending over a portion orannular strip portion. A groove 35 is formed along the juncture of thebend in order to decrease the rigidity of such connection, reducing thestress on the connection of lip 34 and the piezoelectric biomorphactuator 28, especially during flexure of the latter.

The piezoelectric biornorph actuator 28 is fixedly clamped along itsinner rim by two ring shaped clamping members 36 and 37. The first ringshaped clamping member 36 is connected to a tubular support sleeve 38,which is in turn mounted along a tubular side to a second tubularsupport sleeve 39, which surrounds a portion of cylindrical sleeve 24.The second clamping member 37 is connected to the second tubular supportsleeve 39. The sleeve-like construction of the second tubular support39, since it carries both the piezoelectric actuator 28 and movable wall26, allows another positioner to position the movable wall withinresonant output cavity 27 independently of the positioning effected bythe piezoelectric actuator 28. Accordingly, the second tubular supportsleeve 39 is connected to a linkage 40, and the linkage is in turnconnected to a second linkage 41 by a rod 42 that extends through anopening in the pole piece flange portion 10. The second linkage 41 isconnected to the mechanical positioner 42 by a rod 43. This linkagemember rides on a guide 44 extending through an opening in the linkageand fastened to a portion of the pole piece 10. Since the elementsconnecting the positioner 50 with the tubular support 39 are rigidrelative to the lightweight and thin movable wall 26, any fiexure of thebiornorph actuator 28 is communicated from the outer rim portion of thepiezoelectric actuator to the movable wall as if the inner rim of thepiezoelectric actuator was absolutely fixed in position.

One electrical lead 45 establishing an electrical path to the electrodesof the actuator is soldered to the middle electrode 31. A portion ofpiezoelectric layer 30 and electrode 32 is cut away to provide access tothe middle electrode. Lead 45 extends through an insulator sleeve member46 mounted within a passage through the pole piece flange 10 to thelower portion of the magnetron housing. An insulator 47 is supportedwithin a passage through the cylindrical wall 7. An enlarged diameterportion of electrical lead 45 extends through this insulator between theinterior and exterior of the magnetron housing. Included in series withelectrical lead 45 is a conductive helical spring 49 which allowsflexure of the piezoelectric biornorph with minimal restraint or pullfrom electrical lead 45. A like electrical lead, not illustrated, isconnected in identical fashion to outer electrode 32. This lead islocated directly behind electrical lead 45, and in the sectional view ofthe figure is not visible. Such structure contains a like helical springand insulating members within the magnetron housing to provide a secondelectrical connection or path between a biornorph electrode and theexterior of the magnetron housing. A third electrical connection is madeto the other outer electrode 33 through the metallic housing walls ofthe magnetron or through the positioning mechanism 42. The secondclamping member 37 and the second tubular support sleeve 39 areelectrically conductive and provide an electrical path between electrode33 to the linkages 40 and 41 and rods 42 and 43, pole piece 10 to wall7.

Since the piezoelectrical biornorph 28 is restrained or fixed inposition along its inner rim, it flexes along its outer rim as afunction of the magnitude of voltages applied across its electrodes.This positions the movable wall 26 to various positions in dependenceupon those applied voltages. Moreover, since the piezoelectric biomorphactuator is fixed along its inner rim to a sleeve like member which isfree to reciprocate or vary its position along the pole piece, as afunction of the movement coupled thereto from the mechanical positioner50, through rod 43, linkage 41, rod 42, the second linkage 40 carryingthe actuator and supports, the wall 26 because it is carried by actuator28 is also positioned relative to the washer shaped dielectric window 25by positioner 50.

The magnetron chamber containing the anode and cathode is evacuated inany conventional manner.

As is conventional, the cathode 14 and the filament are connected to aconventional electrical connector assembly, not illustrated, which ismounted to the top of the cup like housing portion 1. Likewise, magnetsand cooling fins are assembled to the illustrated structure in theconventional manner. Such conventional elements and mounting bracketsare illustrated in US. Patent 3,034,014 to I. DreXler.

As is conventional, a source of high voltage, not illustrated, isadapted to be connected to the appropriate terminals of the electricalconnector to establish an electric field between the cathode 13 andanode 15, while the magnets, previously discussed, with pole pieces 10and 12 establish a magnetic field in the interaction region or gapperpendicular to the direction of such electric field. A conventionalsource of filament voltage, not illustrated, is adapted to be connectedto the filament through the electrical connector in order to stimulatethe emission of electrons from the cathode 13.

The operation of a coaxial magnetron is conventional and is described inthe literature. In essence, under the interaction of the crossedelectric and magnetic fields, e.g., the electric field extending betweenthe cathode 13 and the surrounding anode 15 across the interactionregion, and the magnetic field extending axially between ends of polepieces 10 and 12 within this same interaction region, potential energyis coupled from electrons emitted by the cathode to an electromagneticwave which appears to travel around the anode at a certain phasevelocity. A TE circular electric mode of oscillation is set up withinthe microwave or resonant output cavity 27. This TE mode has a fixedpositioned phase magnetic field extending around the outer wall 17 ofanode 15. The coupling slots 20 couple this microwave energy from cavity27 to alternate anode resonators, which are thus placed in the sameelectrical phase. Adjacent anode resonators not coupled to the outputcavity have voltages induced from the electromagnetic Waves introducedwithin the output cavity coupled anode resonators which are 180 degreesout of phase with that in the cavity coupled resonators. Thus, betweenany two adjacent anode resonators there is a forced 180 degree shift inelectrical phase. This is the commonly termed 1r mode of oscillation.Since a magnetron is capable of operating in many different modes, it isnecessary to select and attempt to maintain operation in only a singlemode, and desirably the 1r mode. The resonant output cavity through thealternate anode resonator coupling tends to lock the magnetron in the 1rmode. This microwave energy generated by the magnetron is transmittedfrom the resonant output cavity 27 through the microwave window 6 to anelectrical load or other equipment.

The frequency of oscillation of the coaxial magnetron is determinedprimarily by the size of the resonant output cavity; and hence, theresonant output cavity is effectively tuned by adjusting the position ofmovable wall 26. Because the resonant output cavity is so much largerthan any of the individual anode resonators, it stores a largerproportion of microwave energy and therefore has a much larger frequencydetermining effect on the magnetron.

The piezoelectric biomorph 28 is connected by the electrical lead 45,the unillustrated lead connected to electrode 32, and the groundedhousing connected to electrode 33 to a source of sweep voltage, notillustrated, which applies a voltage between the electrodes 31, 32, and33. In accordance with the well known principles of operation of apiezoelectric sandwich, the outer rim of the biomorph is positioned as afunction of magnitude of the applied voltages. Since movement of theactuator 28 is coupled with movable wall 26, this likewise positionsmovable wall 26.

Positional changes or movement of the movable wall effect changes in thefrequency of resonance of cavity 27 over a bandwidth of perhaps fivepercent. However, the biomorph actuator is capable of responding ormoving at a very fast rate from DC up to 1 megacycle of applied sweepvoltages; and hence, tunes cavity 27 over this bandwidth at a very highsweep rate.

The conventional mechanical or electromechanical positioner 50 providesa driving force through rods 42 and 43, and links 41 and 40, the latterof which carries the piezoelectric biomorph actuator 28 and the movablewall 26, and which in the customary manner produces a periodicreciprocating motion. This periodically reciprocates the position ofmovable Wall 26 relative to the dielectric window 25 to cause a changein tuning over a relatively wide band-width; however, due to obviousmechanical limitations, this tuning change or sweep occurs at a smallerrate than that available with the piezoelectric actuator. Sweep tuningwith both the positioner 50 and actuator 28 is performed simultaneously,or in the alternative, the mechanical tuner is utilized merely to setthe initial position of movable wall 26, and frequency sweeping isaccomplished in the foregoing manner solely through the action of thebiomorph actuator 28.

It is noted that the biomorph sandwich 28 is not directly exposed tomicrowave energy from the resonant output cavity, but is in fact,isolated therefrom by the movable wall 26. Thus, the piezoelectricmaterial and the epoxy used to commercially form the biomorph sandwichconstruction which is relatively lossy at microwave frequencies is notheated by the microwave energy appearing in cavity 27. Likewise, sincethe piezoelectric biomorph actuator 28 is not located within theevacuted regions of the magnetron no special constructions, such as theuse of low vapor presure materials in the sandwich is preferred orrequired, and in fact, any ordinary biomorph construction, as iscommercially available, can be utilized. Moreover, in some manufacturingprocedures in which the magnetron is heated to very high temperaturesabove the Curie temperature of the piezoelectric material utilizedduring the evacuation proces, the piezoelectric biomorph need not bedirectly exposed to such temperatures, and can in fact, besubsequentially assembled to the magetron housing. In like manner, anypiezoelectric biomorph found defective in service, since it is locatedoutside the evacuated region of the magnetron, is more easily removedand another piezoelectric biomorph substituted in its place withoutdestroying the vacuum within the evacuated portions of the magnetronhousing.

Not so apparent is the increased rate of tuning obtainable by having themovable wall portion of the tuner confronting and movable relative to adielectric window contained within the microwave cavity. Experimentalresults verified by subsequent calculation showed that only aboutone-fourth as much displacement of movable Wall 26 is necessary toeffect the same predetermined change in tuning of the microwave orresonant output cavity 27 as is required without such a window.

The mechanism for this achievement is believed to operate as follows:Normally the TE circular electric mode of oscillation has a distributionor envelope of the magnitude of the electric field component along theanode wall between the top wall 2 and movable wall 26 that approximatesa half-wave sinusoid. Where movable wall 26 is in a position very closeto the dielectric window 25, the lowest intensity of this half-wavesinusoid distribution is incident upon the dielectric window. Thedielectric window 25 appears as a shunt capacitance to the microwaveenergy and the magnitude of the capacitance has an effect upon thefrequency of resonant of the microwave cavity 27 just as the size of thecavity affects the tuning thereof. Unlike a lumped tuned circuit, commonat low radio frequencies, however, positioning of a dielectric elementwithin a cavity having a distributed electric field has a definiteeffect. The larger the magnitude of the electric field incident upon thedielectric element, then the greater is the capacitive eflYect.

Thus, as the movable wall 26 is positioned more distant from thedielectric window 26, to enlarge the size of microwave cavity 27 andlower the frequency of resonance of the cavity, the TE mode at this newfrequency would result in a spreading out of the halfwave sinusoiddistribution between the movable wall 26 and the top wall 2. However,since the position of dielectric window 25 is fixed, it is exposed tothe electric field at a point of higher intensity along the half-wavesinusoidal field; and thus more electric field is shunted by thedielectric window, which results in a larger effective capacitance. Asis apparent, increasing the capacitance of the cavity serves to furtherlower the frequency of resonance of that cavity.

The opposite effect occurs as the movable wall progresses from a remoteposition to a close position relative to the dielectric window 25.

Thus, there is a cumulative effect due to the positioning of movablewall 26 that results in a greater amount of tuning of about quadruplethat previously available for a given displacement of the movable wall.A necessary consequence of this improvement is that for a given amountof displacement of the movable wall, the bandwidth of frequencies overwhich the resonant cavity is tuned is greater. In the alternative, for agiven bandwidth of tuning the rate at which the cavity is tuned isquadrupled since the wall 26 need be moved only one-fourth the distanceat a given speed relative to previous movable wall type tuners to eifecta sweep over the desired bandwidth.

Of course it is understood that this invention is not restricted to theparticular details as described above, as many equivalents will suggestthemselves to those skilled in the art. The foregoing embodiment, it isunderstood, is presented solely for purposes of illustration and are notintended to limit the invention as defined by the breadth and scope ofthe appended claims.

What is claimed is:

1. A tunable coaxial magnetron comprising:

(A) a cylindrical cathode (B) a coaxial cylindrical anode surroundingsaid cathode and containing a plurality of anode resonators spaced aboutand facing said cathode,

(C) said cathode and said anode resonators spaced apart across aninteraction gap within an evacuated housing;

(D) a coaxial chamber within said evacuated housing,

surrounding said anode, including:

(A) a plurality of conductive walls, and;

(B) an annular shaped dielectric window pervious to microwave energy andimpervious to air to maintain the vacuum in said chamber:

(E) an annular shaped movable conductive wall outside said evacuatedhousing confronting said dielec tric window defining with said chamber amicrowave cavity;

(F) microwave passage means connected between alternate ones of saidplurality of anode resonators and said coaxial chamber for introducingmicrowave energy from said cavity to said alternate resonators.

(G) means for establishing a crossed electric and magnetic field withinsaid interaction gap between said cathode and said anode resonators.

(H) out-put coupling means for passing microwave energy out of saidchamber including a second dielectric window; and,

(I) positioning means connected to said movable wall for adjustablypositioning said movable wall toward or away relative to said dielectricwindow to vary the effective size and resonant frequency of saidmicrowave cavity.

2. The invention as defined in claim 1 wherein said annular shapeddielectric window and said annular shaped movable wall are washer-likein shape.

3. The invention as defined in claim 1 wherein said positioning meanscomprises:

(A) a fixed support;

(B) piezoelectric means connected between said movable wall and saidfixed support and responsive to applied voltages for varying theposition of said movable wall; and,

(C) electrical lead means for connecting said piezoelectric means to asource of control voltage.

4. The invention as defined in claim 3 wherein said piezoelectric meanscomprises a piezoelectric bimorph.

5. In a tunable microwave generating tube which contains means forgenerating radio frequency energy in the microwave region, and at leastone resonant cavity having a portion maintained in vacuum fordetermining the frequency of operation of said means, the improvementwherein said resonant cavity comprises;

(A) an evacuated air impervious metallically walled chamber having atleast one dielectric window for preventing the passage of airtherethrough into said chamber and permitting the passage therethroughof microwave energy;

(B) at least one outer conductive wall movably mounted proximate to andconfronting said dielectric window for reflecting incident microwaveenergy back through said window and defining a boundary surface to saidresonant cavity, and

(C) positioning means for adjusting the position of said movable walltoward or away relative to said dielectric window to vary the effectivesize and resonant frequency of said resonant cavity.

6. The invention as defined in claim 5 wherein said positioning meanscomprises:

(A) a fixed support means;

(B) a piezoelectric means;

(C) means connecting said piezoelectric means between said fixed supportmeans and said movable wall, said piezoelectric means responsive toapplied voltages for positioning said movable wall,

(D) electrical circuit means; and

(E) means for connecting said first electrical circuit means across saidpiezoelectric means for applying voltages from a control voltage sourceto said piezoelectric means.

7. The invention as defined in claim 6 wherein said piezoelectric meanscomprises a piezoelectric bimorph.

8. A tunable coaxial magnetron, comprising: a cylindrical cathode and acylindrical anode, said anode having a plurality of vanes containedthereon facing said cathode across an interaction region and spacedabout said cathode to form a plurality of anode resonators; means forestablishing a crossed electric and magnetic field within saidinteraction region; a chamber maintained in vacuum surrounding the outerside of said anode; a plurality of slots contained in said anode forconnecting said chamber with alternate ones of said plurality of anoderesonators; a Washer-like shaped dielectric window bordering saidchamber, said washer-like shaped dielectric window being pervious tomicrowave energy and impervious to air for maintaining a substantialportion of said chamber in vacuum; a movable washer-like shaped wall ofconductive material located outside said chamber and confronting saiddielectric window; said movable washer shaped wall together with saidchamber defining a resonant output cavity; washer shaped piezoelectricmeans connected to said movable wall and responsive to electricalcontrol voltages for positioning said wall as a function of said controlvoltages toward References Cited UNITED STATES PATENTS Bach 331155 XKroger 315-39.55 X Drexler 315-39.77

McLeod SIS-39.61 X Truax 315-39.77 Buck 315-3977 X Plumridge 315-39.55

5 HERMAN KARL SAALBACH, JR., Primary Examiner s. CHATMON, JR., AssistantExaminer US. Cl. X.R.

Linder 332 5 X 310-8;31539.61,39.77;33190, 15s

g ggy UNITED STATES PATENT OFFlCE CERTIFICATE OF CORRECTION Patent No.3, 147 2 x7 Dated Novemt er 1 I 1969 Inventor(s) Joseph F. Hull It iscertified that error appears in the above-identified patent and that.said Letters Patent are hereby corrected as shown below:

In Column 5, bridging lines b3 and M the phrase "conductive layer ofelectrode" should read --conductive layer or electrode--; in Column 5,line 6b, the phrase "the outer electrode" should read --the other outerelectrode--; in Column 7, line 2 3, the word 'positioned" should read--positional--.

SIGNED AND SEALED Attest:

Edward M. Fletcher, Ir. Wml'flf E. SGHUYL m, Attesting Officer I 551011of Patents

